The dialogue text field for an NPC becomes uneditable once you enter too much text into it.

Steps to Reproduce

1. Summon an NPC
2. Open the NPC interface in creative mode
3. Edit the dialogue text field to contain lots of text (I had 18 lines of text)
4. Exit out of the text field

Expected Outcome
The text field should still be accessible and allow editing

Actual Outcome
The text field is no longer accessible and prevents editing